The present invention relates to modulation control in FM broadcasting, and more particularly to a system for automatically selecting the gain of a multichannel modulating signal in accordance with the use or non-use of the various component channels thereof.
In FM broadcasting, a carrier signal is modulated by a composite modulating signal which is comprised of a number of signals in separate frequency channels. Various SCA channels may be included in addition to the (L + R) and (L - R) channels associated with stereophonic FM broadcasting. The maximum amplitude level of this composite signal must be controlled in some manner so that modulation constraints are not exceeded. It will be noted, however, that this maximum amplitude level will be dependent upon the use or non-use of the various frequency channels, and will be greatest when all available frequency channels are in use. To prevent overmodulation in this circumstance, it has previously been the practice to adjust the gain of the composite signal when all channels were being used. In this event, however, the modulation level will necessarily decrease when various channels are deleted for one purpose or another. This can result in as much as a 30% reduction in peak modulation. Preferably, however, the peak modulation level should be maintained at as high a level as possible so as to provide the maximum degree of signal clarity and quality in subsequent reception of the modulated signal.
It is therefore desirable to provide a system which automatically selects the gain of the composite modulating signal so as to maintain peak modulation at a constant 100% in all modes of operation.
In accordance with the present invention, a plurality of input sources additionally generate binary control signals indicative of the current use or non-use status of the corresponding channel for the purpose of controlling the gain of a composite modulating signal. A signal summer is provided which combines the various input signals to provide a composite signal. The summer has a gain which is selectable in accordance with control signals supplied to gain select inputs thereto. The control signals which are supplied by the various input channels are each directed to a corresponding gain select input of the signal summer so that the gain of the composite signal is automatically selected in accordance with the use status of the various channels.